X-ray tube energizing circuit



April 4, 1950 H. M. SMITH .XRAY TUBE ENERGIZING CIRCUIT Filed Feb. 15,1947 AMPLIFIER AMPLIFIER Z2 REACTOR I2 SAMPLE FILTER AMPLIFIER VOLTAGEREGULATOR AC. SOU RC E h, w nm n 3 ,w n m 6M A m 5 Patented Apr. 4, 1950I X-RAY TUBE ENERGIZING CIRCUIT Henry M. Smith, Schenectady, N. Y.,assignor to General Electric Company, a corporation of New YorkApplication February 15, 1947, Serial No. 728,851

2 Claims.

My invention relates to X-ray tube energizing circuits, and moreparticularly to an automatically stabilized energizing circuit for anX-ray tube whereby the intensity and the hardness of X-rays produced bysuch tube remains essentially unchanged for usual variations in voltageof the energizing source.

X-rays have become a valuable means for investigating the internalstructure and physical characteristics of otherwise opaque materials.Other similar uses are also known in the art. In conducting theseinvestigations, it is usual to employ means for measuring the intensityof the unabsorbed X-rays emanating from the material being analyzed.When employing such a method, the accuracy of investigation is dependentupon the accuracy with which the intensity and the hardness of thepenetrating X-ray beam may be held constant.

An X-ray tube is usually employed to produce the X-ray beams utilized ininvestigation of materials of the type described. The characteris-.

to-cathode voltage is dependent upon the temperature at which itscathode is maintained. Also,

the equivalent wavelength or hardness of the X-rays produced by a tubeenergized by an A.-C. voltage is critically dependent upon the magnitudeof the anode-to-cathode voltage. If this voltage is increased, thewavelength becomes shorter and the penetrating power of the X-raybecomes greater. This efiect is most pronounced in the longwavelengthregion which region is preferably employed in analyses ofmaterials whenever feasible, such as in analysis of materials of suchstructure and dimensions which will absorb less than all of such longerwavelength X-rays. Thus, it has been found that a one per cent increasein the X-ray tube anode-to-cathode voltage may, in some instances,increase the amount of unabsorbed X-rays passing through a materialunder test as much as twenty per cent or more. Similar effects uponunabsorbed X-ray intensity may be caused by changes inthe cathodefilament emission. It is desirable, therefore, if accurate measurementsare to be obtained, that the X-ray tube be subjected to voltages whosemagnitudes are controlled with a high degree of It is another object ofmy invention to provide regulating means responsive to the intensity ofthe X-rays produced by an X-ray tube whereby variations in filamentemission are minimized.

Another object of my invention is to provide an X-ray tube energizingcircuit of such type that the intensity and hardness of the X-raysproduced thereby is maintained constant with a high degree of accuracy.

The features of the invention which are believed to be novel andpatentable will be pointed out in the claims appended hereto. For abetter understanding of the invention, reference is made in thefollowing description to the accompanying drawing in which the singlefigure thereof is an embodiment of my invention in which an energiz ingcircuit for an X-ray tube is disclosed.

Referring now to the drawing, X-ray tube l possessing anode 2 and afilament-type cathode 3 is employed as a source of X-rays whoseintensity-and hardness are to be accurately controlled. X-ray tube l ispreferably placed within an absorbing shield, partially disclosed at 4,which shield possesses appropriate openings 5 and 6 through which X-raybeams may emanate. Openings 5 and 6 may be made at convenient points onshield 4, such that the emanating X-rays may either be of equalintensity or of intensities of constant ratio with respect to eachother.

X-ray tube I may be energized from a suitable A.-C. source I, of theusual type, in which the magnitude of the supply voltage, as expressedin effective or R. M. S. value, may be subjected to both gradual andrapid variations because of changes and disturbances in its connectedload other than the X-ray tube under consideration. A voltage regulator8 of conventional type is preferably used to remove the greater portionof changes in the effective'value of the A.-C. source by connecting itto source 1 through a suitable switch 9, and utilizing its outputterminals as a voltage source. Cathode 3 of X-ray tube l is energizedfrom the output terminals of voltage regulator 8 through an isolatingtransformer l0, whose secondary'll is connected directly across thefilament of cathode 3 and whose primary I2 is preferably connected tothe output terminals of a tapped auto-transformer [3 with a variableimpedance of a type to be described in series therewith. The inputterminals of transformer l3 are connected across the output of regulator8. Tap changing switch M is provided whereby the voltage ratio ofautotransformer 13 may be manually adjusted.

"crease.

A high voltage step-up transformer 15 is employed in the followingmanner to provide the necessary anode-cathode voltage. Anode 2 isconnected to ground, and cathode 3 is connected to one side of the highvoltage secondary it of transformer l5 by connecting one side ofsecondary i! of the isolating transformer it to one terminal ofsecondary It. The remaining terminal of secondary i6 is connected toground through an anode current measuring device ii and a tappedresistor id in series therewith. The primary [9 of transformer I5 isenergized from the output terminals of a tapped auto-transformer Zllthrough a variable impedance to be described. A manually operated tapchanging switch *2! is also provided whereby the ratio ofauto-transformer 29 can be adjusted. Auto-transformer 2B is alsoenergized from regulator 8 as is auto-transformer it.

The anode current of X-ray tube 1 for a constant anode-to-cathodevoltage is dependent upon the filament emission of cathode 3. Suchfilament emission can be held constant by properly controlling thevoltage impressed across pri* mary l?! of transformer lfl. The voltageacross primary l2 can be regulated by placing in series therewith anadjustable impedance such as a saturable reactor 22 including A.-C.windings 22' and a D.-C. winding 22" and regulating the D.-C. excitationsupplied thereto.

Since anode current exists in X-ray tubes only when the anode ispositive with respect to the cathode, a pulsating D.-C'. voltage appearsacross resistor it when the X-ray tube is energized by an A.-C. source.A portion of this voltage can be utilized by providing a tap 23 onresistor it. This voltage has a D.-C. component which is positive withrespect to ground for a circuit such as disclosed herein and isessentially directly proportional to filament emission at constantanodeto-cathcde voltage. The voltage appearing at tap '23 is impressedupon the input terminals of a direct current amplifier 24 ofconventional type whose output terminals are utilized as a source ofdirect current whose value is inversely proportional to the positiveD.-C. component of input voltage. The amplifier may or may not includefiltering means to eliminate A.-C. components. In either case the D.-C.saturating current is the same, and A.-C. components, if present, havelittle effect upon the reactor. Thus, energization of the direct currentwinding on saturable reactor 22 from amplifier 2d causes the impedanceof reactor 2? to increase if the anode current of tube l tends toincrease, and correspondingly to decrease if the anode current of tube ltends to de- Thus, for a constant cathode-to-anode voltage, the filamentemission is maintained essentially constant without respect tovariations in effective value of the A.-C. voltage appearing across theoutput terminals of auto-transformer I3.

The anode-to-cathode voltage of tube l is maintained constant in thefollowing manner. An X-r ay detector 25, responsive to the intensity ofX-rays reaching it, is placed in such position with respect to X-raytube i that the X-ray beam' emanating through opening 3 impinges uponthe detector. Since X-ray detectors, particularly those of a type suchas a Geiger-Muller counter or phctotube possessing a fluorescentcoating, detect intensity variations, it is desirable to utilize them ina system designed to cause their operation in a region where variationsin specimen thickness and in energizing voltage values cause maximumX-ray intensity variation. This can be accomplished by utilizing afilter 26 of X-ray absorbing material, preferably essentially equivalentin total absorption to that of the specimen to be analyzed, and placingit between the X-ray tube 1 and the detector 25. The intensity ofunabsorbed rays reaching the detector is then a function of thepenetrating power or hardness of the original rays. Detector 25 andfilter 26 together thus constitute an X-ray hardness responsive device.The output of detector 25 is amplified and, converted into a directcurrent whose value varies inversely with the output of detector 25 byutilizing an amplifier 21 of appropriate conventional type. A saturablereactor 28, including A.-C. windings 28' and a D.-C. winding 28'', whoseD.-C. winding is energized by the output of amplifier 2?, is employed asa variable impedance in series with primary 19 of transformer [5 acrossthe output terminals of auto-transformer 2E3. Since the impedance ofreactor 28 varies inversely with the amount of direct currentmagnetizing efiect supplied by amplifier 2? to D.-C. winding 28" thevoltage across the primary !9 is automatically maintained at that valuewhich causes X-rays of desired intensity to be produced by X-ray tube2'. Since it is desirable to operate both reactors 22 and 28 normally insuch fashion that either an increase or a decrease in their effectiveimpedance is obtainable with equal ease, direct current measuringdevices 29 and 30 preferably are provided in series with the D.-C.windings 28 and 22 of reactors 28 and 2?. respectively, whereby normaloperation at the desired point on their impedance characteristic may beassured.

Utilization of X-ray tube l as a source of X-rays for inspectionapparatus is illustrated by showing a material 3!, to be analyzed,intercepting the X-ray beam emanating through opening 5 of shield l, anX-ray detector 32 subject to the unabsorbed X-rays passing throughmaterial 3i, an amplifier 33, and a meter 34 connected to detector 32through amplifier 33 in such fashion that the intensity of theunabsorbed rays may be directly indicated on meter 34.

The operation and advantages of the circuit shown in the drawing may bemore fully described in the following manner. When inspecting a sample3! it is desirable that the X-ray tube i provide X-rays of suchintensity and wavelength that the unabsorbed X-rays impinging upondetector 32 are of such value that the indication of meter 3 is afunction only of the thickness and characteristics of sample 3! The voltage applied to X-ray tube l between anode 2 and cathode 3 and thevoltage applied across the filament of cathode 3 are made adjustable sothat X-rays of appropriate intensity and hardness to produce aconveniently readable m'eter indication may be produced.

When the circuit is energized from source 7 by closing switch 9, avoltage dependent upon the setting of contact switch l4 and upon thevalue of impedance represented by the saturable reactor 22 is impressedacross'the filament of cathode 3. Similarly, a voltage-dependent uponthe setting of contact switch 2! and upon the value of impedancerepresented by saturable reactor 23 is impressed between cathode 3 andanode 2. Adjustment of contact switches M and 2! preferably should bemade essentially simultaneously and in a manner to be described.

Since X-ray detectors 25 and 32 are preferably of similar design and aremade to operate most efficiently at the same low'X-ray intensityl'evel,a filter 26;w-hose total-absorption of X-raysis essentially equivalentto that of sample 3|, is interposed between X-ray tube l and. the X-raydetector 25. The anode-cathode voltage can then be adjusted by manuallypositioning contact switch 2| until the unabsorbed X-rays impinging upondetector 25 are of the desired intensity as evidenced by meter 29 whilealso adjusting contact l4 to obtain or maintain the desired value ofanode current as indicated by meter H. The desired X-ray intensity isthat value which will cause amplifier '21 to deliver a direct current tothe D.-C. winding 28" of reactor 28 to partially saturate reactor 28 tosuch an extent that decrease or increase of the saturating directcurrent causes a corresponding increase or decrease of impedance of theA.-C. winding of reactor 28.

If the desired anode current causes amplifier 24 to supply an impropervalue of D.-C. current to reactor 22, as evidenced by meter 30, the tap23 on resistor 18 can be changed to correct this condition. Additionaladjustment of contact switch l4 may then be required since a change inposition of tap 23 changes the impedance represented by the A.-C.winding 22 of reactor 22. In this fashion, X-ray tube I may be caused tooperate at the desired intensity and equivalent Wavelength with reactors22 and 28 normally excited by direct currents of desired value.

Whenever undesirable voltage fluctuations of source 1 are caused byextraneous conditions, voltage regulator 8 compensates for suchvariations but only to an extent dependent upon its characteristics.across auto-transformers l3 and 20 contains objectionable componentswhenever disturbances on source I occur.

When the voltage across transformer 20 increases, the anode-to-cathodevoltage of X-ray tube I tends to increase correspondingly. Such anincrease ofanode-to-cathode voltage materially increases the penetratingpower of X-rays produced, and X-ray detector 25 is immediatelyinfluenced thereby. Amplifier 21, as controlled by detector 25,correspondingly decreases the D.-C. excitation to reactor 28 whichthereby increases its value of impedance which is in series with primaryl9 of transformer I5. Such an increase in impedance when the voltageacross r transformer 20 tends to rise compensates for such rise, and thevoltage across winding i9 is thereby maintained essentially constantregardless of voltage variations across transformer 20. Conversely, whenthe voltage across transformer 20 decreases, the impedance of reactor 28decreases. Similarly, the filament emission or anode current, asindicated by meter I1, is maintained essentially constant since thevoltage across primary l2 of transformer I0 is essentially isolated fromvoltage variations across transformer i3 by the automatic change ofimpedance value of the A.-C. winding 22 of reactor 22 in seriestherewith. Thus, the intensity and equivalent wavelength of the X-raysproduced by X-ray tube l are held constant with a high degree ofaccuracy, independent of expected disturbances in the X-ray tubeenergizing source.

Since filters of any desired total X-ray absorption may be substitutedfor filter 26, it is evident that the X-ray tube may be operated at anydegree of intensity and at any equivalent wavelength at or below itsmaximum rating, while still retaining the automatically controlledfeatures of my invention.

Thus, the voltage impressed "'As'will 'occur 'to' those skilled in theart, varione different arrangements and combinations of the principlesdescribed above may be employed without departing from the true spiritand scope of the invention and I, therefore, do not wish to limit myinvention to the particular arrangement described. v

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

value of direct current in the D.-C. winding of said reactor, an X-rayabsorbing filter positioned to intercept X-rays emanating from one portof said X-ray tube, an X-ray detector to provide an electric currenthaving a value which varies with variations in the penetration of X-raysthrough said absorbing filter, and an amplifier' connected in responsiverelation to said detector and in energizing relation to the D.-C.winding of said reactor to provide through such winding a value ofdirect current which varies in inverse relation to values of X-raypenetration detected by said detector, whereby the penetrating power ofX-rays produced by said tube is maintained substantially constant.

2. In combination, a two-port X-ray tube having an anode and a cathode,said cathode comprising a filament which is heated bythe passage ofelectric current therethrough, a resistor, first and second transformerseach having a primary and a secondary, the secondary of said firsttransformer being connected to said filament, the secondary of saidsecond transformer being connected in series with said resistor betweenthe anode and the cathode of said X-ray tube, energizing means to applyalternating current to the primaries of said first and secondtransformers, first and second saturable reactors each having an A.-C.winding and a D.-C. winding, the A.-C. winding of said first reactorbeing connected in series between said energizing means and the primaryof said first transformer, the A.-C. winding of said second reactorbeing connected in series between said energizing means and the primaryof said second transformer, an X-ray absorbing filter positioned tointercept X-rays emanating from one port of said X-ray tube, an X-raydetector to provide an electric current having a value which varies withvariations in the penetration of X-rays through said absorbing filter, afirst amplifier connected in voltage-responsive relation across saidresistor and in energizing relation to the D.-C. winding of said firstreactor to provide through such winding a value of direct current whichvaries in inverse relation to current through the anode-to-cathodecircuit of said X-ray tube, and a secondary amplifier connected inresponsive relation to said detector and in energizing relation to theD.-C. winding of said second reactor to provide through such winding avalue of direct current which varies in inverse relation to values ofX-ray penetration detected by said detector, whereby both the intensityand 7*" 8 tha-penetratingrpowenxsofutxrayspztoducedmby Number Name?Date.- said tube. are maintained=zsubstantially constant. 2,151,602Kearsley Mar..:21,,1939

I HENRY M. vSMITH. 2,156,074 Westendorp April'25, 1939 2,160,605 SuitsMay 30, 1939- REF NC C I 5 2,217,939 Bischoff Oct. 15,.1940 Thefollowing-references areaof record inthe 2,222,451 Trost' NOV 19, 4 fileof this patent: 2,319,373 Weisglass y 1943 V 2,401,289 Morgan et' a1.May 28, 1946: UNITED STATES PATENTS 2,404,905 Garretson July 30, 1946Nmnber Name Date 10 1,876,437 Wantz Sept. 6,1932 OTHER REFERENCES2,094,318 Fai a Sept 1937 G. E. Review, vol. 48, No. 3, March 1945. 9

2,097,760, Failla NOV. 2, 1937

